From the preface to Let the Nations Be Glad (1st edition):
This book is a partial payment of a debt I owe to the nations. The apostle Paul is not alone in saying, "I am a debtor to the Greeks and to the Barbarians, to the wise and to the foolish" (Romans 1:14). To those culturally near me and those culturally far I am a debtor. Not because they gave me anything that I must pay back, but because God gave me what can't be paid back. He gave me the all-satisfying pleasure of knowing him and being loved by him through his Son Jesus Christ.
What makes a debt a debt is that if you don't pay it, you lose a possession. They take back your house or your car. And the more precious the possession, the more urgent the payment of the debt. If I don't do my utmost to show the nations "the light of the knowledge of the glory of God in the face of Christ," I will in effect be saying, "It is not infinitely valuable. It is not absolutely necessary for eternal life. It is not great enough to satisfy the deepest needs in every culture on earth. And it's beauty has not freed me to be a man for others." But if I say this, then I do not believe in "the surpassing worth of knowing Christ Jesus my Lord." And if I do not believe, I lose everything. Therefore I am a debtor. For I would rather lose anything and anyone on earth, than to lose Christ.